Remove PluginLoadObserver and related logic, it was only used for NPAPI.

content/plugin/webplugin_proxy.cc

Index: content/plugin/webplugin_proxy.cc
diff --git a/content/plugin/webplugin_proxy.cc b/content/plugin/webplugin_proxy.cc
index 22e1890b031385027637ca4518b34a0db2b2f6e7..00190c7a8cedd784049e498b3bf3cbc14ae4c012 100644
--- a/content/plugin/webplugin_proxy.cc
+++ b/content/plugin/webplugin_proxy.cc
@@ -275,7 +275,6 @@ void WebPluginProxy::HandleURLRequest(const char* url,
                                       const char* target,
                                       const char* buf,
                                       unsigned int len,
-                                      int notify_id,
                                       bool popups_allowed,
                                       bool notify_redirects) {
   if (!target && base::EqualsCaseInsensitiveASCII(method, "GET")) {
@@ -303,7 +302,6 @@ void WebPluginProxy::HandleURLRequest(const char* url,
     memcpy(&params.buffer.front(), buf, len);
   }
 
-  params.notify_id = notify_id;
   params.popups_allowed = popups_allowed;
   params.notify_redirects = notify_redirects;